Document 2349433

J. Mater. Environ. Sci. 3 (6) (2012) 1009-1018
ISSN : 2028-2508
CODEN: JMESCN
Sharma et al.
Corrosion Investigations Due to Increased Sulfidity in Digester House of
Paper Mill
A. Sharma 1, Sulaxna 2, A.K. Singh 3
1
Associate Professor, Department of Physics Graphic Era University Dehradun, India
2
Assistant Professor, Department of Chemistry THDC, IHET Tehri Garhwal, UK,
3
Professor, Department of Paper Technology, Saharanpur Campus IIT Roorkee, India
Received 1 Mar 2012, Revised 24 July 2012, accepted 24 July 2012.
Corresponding Author: [email protected]
Abstract
Increased demand of paper, pollution control and better energy management are the driving forces for the
development of the paper industry aspiring to enter 21st century in a big way. High sulfidity pulping of wood is
one of the approaches in this direction, which attacks lignin in a more selective way thereby increasing pulp
yield and makes it possible to bleach pulp to higher degree of brightness with less pollution load. Higher
sulfidity has been found to affect corrosion of the digester house and related machinery and through this a
question mark on the credibility of materials of construction. Citing example of a typical paper mill, which
increased sulfidity in their pulping procedure, attempts have been made to analyze the increase in corrosivity
due to changed concentrations of sodium sulfide, polysulfide and thiosulfates in the cooking liquor of the mill
in question. E-pH diagram, popularly known as the Pourbaix diagram, have been constructed for S-H2O and
Fe-S-H2O system with relevant concentrations of the different type of sulfur species and have been used in this
endeavor.
Key Words : Sulfidity, Corrosivity, Pourbaix diagrams, Pulp and Paper
1. Introduction
Pulp and paper making technology has undergone rapid changes during the last decade. The factors behind
these changes have been pollution control strategy, better energy management and increased production
alongside improvement of quality of paper. The purpose is to meet the ever growing need of paper and paper
products in the lowest possible cost and more importantly, without affecting the eco-balance of nature.
Perhaps the strongest driving force in the modification of paper making process is to develop and adopt new
technology which are environment friendly and do not pollute water as well as air. In this endeavor, changes
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being adopted are with regard to pulping and bleaching practices in pulp and paper industry. To a large extent,
bleaching is solely responsible for enhancing colour and increasing BOD,COD, dioxin etc. level of water
stream to which discharge of paper mill are thrown while recovery boilers are responsible for bad odour and
air pollution of the surrounding environment. To minimize and ultimately bring down pollution to ‘ZERO’
level from both of these sources, the normal practice has been (i) to change over to elemental chlorine free
(ECF) or total chlorine free (TCF) bleaching and (ii) modification in the pulping procedure such that
requirement of bleach chemicals (indicated by ‘kappa’ no.) drops to lower levels, which also helps in bringing
down pollution level.
One most likely modification in pulping process is applicable process is application of rapid displacement
heating (RDH) or super batch cooking [1, 2]. In this process, cooking of chips (delignification) is done with
cooking liquor of higher sulfidity, which accelerates delignification because it more selectively attacks lignin.
The result is that (i) pulp strength is improved (ii) pulp yield is = 1.5-2% higher (iii) pulp has low kappa no.
(less demand of bleach chemicals and hence less pollution). This process also shows better heat economy and
better brown stock washing thereby improving recovery of chemicals also. Since most sulfide is removed in
the pulping stage itself, decreased sulfide in black liquor has advantage in reduced corrosion of recovery
systems and also less TRS (total reduced sulfur) emissions from recovery boilers (TRS)emission is responsible
for bad odour. However, increased sulfidity in cooking liquor affects corrosion of digesters and related plant
machinery thereby cutting short their useful life drastically [3]. The present paper deals with this aspect of
operation in digester house.
2. Chemicals and Cooking Liquor :
Paper industry uses ‘kraft’ process most widely for pulping of wood (Table-1).
Table 1: Pulping Process (Global Data)
Category
Mechanical Pulp
Chemical Pulp Kraft
Sulfite
Neutral Sulfite Semi Chemical NSSC, etc
Amount (metric tone)
31x106
88 x 106
9 x 106
8 x 106
According to above, share of ‘kraft’ process is about 84% of all the chemical pulping and 64% of the total
pulping processes. The cooking liquor in kraft pulping consists mainly of NaOH and Na2S. The pH of this
solution is 13.2-14 at room temperature. In addition, the liquor also contains (i) Na2SO4 (added as a make up
chemical in recovery boiler as ‘salt cake’) due to its incomplete reduction in furnace (ii) Na 2CO3 (formed in
furnace), due to incomplete causticizing (iii) Na2S2O3, due to air oxidation of sulfate (iv) Na2Sx+1, formed due
to following equilibrium:
Na2S + Na2S2O3 = Na2Sx+1 + X. Na2SO3
and (v), as contaminant from wood, fresh water or recycled filtrates in the process. Average composition of
typical cooking liquor from mills in Western countries and from Indian mills is given Table 2.
The amount of sulfidity in cooking liquor of US mill is ~ 40% while that of Indian mill is ~20%. This
corresponds to 0. 7gm mole/liter and 0.2 gm mole/liter of total dissolved sulfur (considering all forms of
sulfur) in former and latter respectively.
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Table 2: Composition of cooking liquor (amount in gpl)
Chemicals
NaOH
Na2S
Na2CO3
Na2SO4
Na2SO3
Na2S2O3
Na2Sx+1
Indian Mills
65
15
25.33
13.5
1.79
2.3
2.4
U. S. Mills
44.7
28
25
9.8
1
6
1.8
2.1. Cases of enhanced corrosion:
Case 1: In one mill [4] a stationary digester fabricated of C-steel was observed experiencing corrosion at a
rate of about 100 mpy to 120 mpy. Cooking liquor corrosion related failures were also observed within 2 years
of operation of white liquor lines of mild steel. This was in spite of the fact that the mill was practicing
operating conditions as per the norms of the industry.
The analysis of liquor showed the presence of 3 gpl of thiosulfate (S2O32-) which was responsible for increased
corrosion. Addition of elemental sulfur (S) to white liquor at concentration greater than 7 gpl was found to
decrease corrosion significantly. Addition of sulfur increases polysulfide concentration in liquor which
decreases corrosion rate.
Case 2: In another mill [5] their batch digesters were found to have been affected by severe corrosion within
ten years of operation. Analysis of cooking and white/green liquor from pulp mill and causticizing plant
respectively showed increase in sodium thiosulfate from 1.9 to 3 gpl while average polysulfide was 2.44 gpl.
Corrosion experiments conducted on white liquor with varying polysulfide concentration to 19 -20 gpl lowers
the corrosion rate to ~ 1.83 mm/year. Obviously the amount of polysulfide in the digester liquor was at a
dangerous level from corrosion point of view. Remedial measures for reducing corrosion included increasing
polysulfide content to a level of 19-20 gpl by either adding Sulfur to white liquor or by oxidizing the white
liquor. This change of Na2S to polysulfides is as per following reactions:
2H2O + 2Na2S + O2 = 2S + 4 NaOH
XS + Na2S = Na2Sx+1
However, the authors feel that (i) corrosion rate of 1.84 mm/year (~73 mpy) at 19 gpl polysulfide level is very
high. Even with this rate, thinning of digester wall will be rapid (ii) localized corrosion may be observed at
higher concentration (> 19 gpl) of polysulfide (iii) long term experiments should also be conducted to know
real corrosion rate and about localized corrosion. Experiments in present work were of 3-4 hours.
Case 3: Another mill increased sulfidity from 21-22% to 26-27%, which has resulted into increased corrosion
of digester house machinery [6]. Following corrosion effects were noticed in a time spam of three years:
- original thickness of inner casing of mild steel circulating pump in digester decreased from 6 mm to < 1 mm
(corrosion rate ~ 67 mpy). Roof and walls of top dome of liquor preheated was affected by pitting. Bottom
plate of dome, where heat exchanger tubes of liquor preheated are welded, is affected by grooving and uniform
corrosion. Other affected portions are joints of liquor heater line with digester, blow lines and liquor lines.
These are the cases where one notices that not only general corrosion, but even localized corrosion can get
initiated with increased sulfidity as one suspects on the basis of effect of polysulfide on corrosivity.
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3. Analysis and Discussion :
Considering concentration of OH- ions cooking liquor is from 0.5 to 2.5 gm mole/liter and the ionization
constant of water at temperature from 250C to 1500C [1] the pH of cooking liquor at temperatures will be as
given in Table 3 below.
Table 3: pH of cooking liquor at different temperatures
Temperature
250C
1000C
1300C
1500C
pH
13.20-14.40
11.96-12.66
11.60-12.30
11.34-12.04
Thus as the liquor is heated during pulping from room temperature to 1500C, its pH varies from 14.4 to 11.34.
These pH ranges will therefore be interest from viewpoint of investigating corrosion in cooking liquor.
E-pH diagrams for S-H2O system were drawn to understand (i) the nature of sulfide species likely to be
present in cooking liquor depending upon the temperature, pH and concentration and (ii) to draw, in turn, FeS-H2O Pourbaix diagram so as to understand the type of iron sulfides likely to form. The diagrams were drawn
for liquors of 20% and 40% sulfidity at room temperature. These diagrams shows that in cooking liquor (with
pH varying 11.6-14.4), sulfide species at reducing conditions will be mostly HS- and only small amount of S2-.
Only at room temperature, where pH of cooking liquor is ~ 14, both HS- and S2- will be in similar amount. In
the presence of oxidants, these species would be in the form of S2O32- and/or polysulfide ions Sn2- with n
varying from 2-5 at temperatures ≤ 1200C as at higher temperatures polysulfides are unstable. Polysulfide has
been found in highly alkaline and hot solutions (case of cooking liquor) [7].
Carbon steel corrodes in alkaline solution (without sulfide) through following reactions:
Fe → Fe2+ + 2e- (anodic)
2H+ + 2e- → H2 (Cathode)
O2 + 2H2O + 4e- → 4OH- (Cathode)
At the concentration of OH-, 0.5 – 2.5mole/liter, prevailing in cooking liquor Fe3O4 is the stable oxide
providing protection [8]. Fe3O4 forms as a result of following reactions:
Fe2+ + 2OH- → Fe(OH)2
Fe(OH)2 + OH- → Fe(OH)33Fe(OH)3- + H+ → Fe3O4 + 5H2O + 2eOn increasing temperature, formation chances of Fe3O4 recedes (On the basis of E-pH diagram) [9, 10] and
HFeO2- or γ – Fe2O3 may form depending upon presence of oxidants (e.g. dissolved oxygen etc.). Both of
these are not as protective as Fe3O4. Hence even in the absence of any sulfide, steel will corrode though at
moderate rate [8]. Presence of Na2S in the cooking liquor increases corrosion rate, because a less protective
film of corrosion products form which consists of oxides and sulfides. The latter may be forming as a result of
following reaction:
Fe2+ + 2HS- → iron sulfide
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Corrosion products formed on carbon steel exposed to sulfide solutions have appearance of black top and
yellow-red bottom close to the metal surface [9]. The former ones appear to be sulfides while yellow-red
coloured products are iron oxides. Also a higher redox potential is required in the solution for the formation of
a protective oxide layer on carbon-steel in the presence of sulfide ions. From Fe-H2O Pourbaix diagram at
room temperature [10], redox potential required for forming Fe3O4 is volt (SHE) while in sulfide containing
solutions, this potential is -0.52 Volt (SHE) (See fig.2 Fe-S-H2O diagram at 250C). When Na2S of white
liquor oxidizes in recovery boiler, it changes to Na2S2O3 according to following reaction:
2Na2S + 2O2 +H2O → Na2S2O3 + 2 NaOH
Presence of thiosulfate in liquor further enhances corrosivity as it helps in accelerating reduction reaction,
given below, which in turn increases corrosion of iron:
S2O32- + 8H+ + 8e- → 2HS- + 3 H2O
Thus reduction of thiosulfate ion will also be accompanied by increase in pH of liquor. In the presence of
thiosulfate, corrosion of iron will be governed mainly by above reaction. Corrosion products formed here will
be iron sulfides.
Thiosulfate also converts into polysulfide as per following equilibrium:
HS- + XS2O3 2- ↔ S1+x2- + XSO32- + H+ (X varying from 2 to 5)
HS- + S2O32- ↔ S
A higher concentration of S2O32- ions along with higher sulfidity will result into formation of S22-,
S32- etc. depending upon concentration.
These polysulfides enhance corrosion, since they undergo following cathodic reactions:
S22- + 2H+ + 2e- → 2HSS32- + 3H+ + 4e- → 3HSFollowing can be seen from above discussion:
a. Formation of higher polysulfide (S32-, S42- etc.) will enhance corrosion more than S22- i.e. higher sulfidity
which results in formation of higher sulfides will end up in forming more corrosive media.
b. Reduction of polysulfides also increases alkalinity of liquor. It is also interesting to note that corrosivity
due to polysulfides is highly dependent upon its concentration. Corrosion rate increases in concentration
of polysulfides up to 2-5 gpl. However, corrosion rate decreases on further increases the amount of
polysulfide beyond 5 gpl (up to 19-20 gpl), by passivating the steel surface. It seems at low
concentration, steel corrodes in active region. On initially increasing concentration, corrosion increases
due to shifting of cathodic polarization curve, representing S22- + 2H++ 2e- → 2HS-, towards anodic
values. Beyond 5 gpl, this curve cuts the anodic polarization curve of iron in passivation region thereby
passivating steel.
Two questions arise here:
a. On further increasing concentration, steel must transit into transpassivation region thereby increasing
the possibilities of pitting and crevice corrosion. It is worth investigating such limits of polysulfide
concentration where steel is likely to experience localized corrosion.
b. What is the nature of sulfides under active and passive corrosion?
In order to know the possibility of forming various sulfides and hence passivation of carbon steels in
cooking liquor, E-pH diagrams for Fe-S-H2O systems were constructed for total dissolved sulfur
content of 0.2gm mole/liter (corresponding to 20% sulfidity) and 0.7 gm mole/liter (corresponding to
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40% sulfidity) at room and high temperature (Figs. 1 to 4). Details about the various equilibrium and
the chemical potentials of various issues are given in ref. [11]. The conditions considered are those
which generally prevail for cooking liquor in digester.
A comparison of these diagrams with those of Fe-H2O system indicates about following aspects:
a. Thermodynamic stability regions of various iron sulfides namely mackinawite, troilite, pyrrhotite and
pyrite exist up to ~0.52 Volts (at pH ~14 and 250C). At more anodic potentials only one expects to
observe protective oxide Fe3O4 in sulfide solutions. In case of carbon-steel in H2O, Fe3O4 is observed
at a lesser anodic potential. Thus corrosion products forming on carbon-steel should be mainly sulfides
with a small amount of oxides. As some of these sulfides namely mackinawite and troilite are of nonprotective type [12, 13] their formation in place of oxides increases the corrosion attack on carbonsteel in sulfide solution. It will be interesting to observe the potential of carbon-steel in sulfide solution
and identity the nature of sulfides formed in such cases.
b. At more active potentials, among the sulfides, one expects to observe mackinawite/troilite. Both of
these are non-protective type and dissolve very easily in solution. As such, as carbon-steel comes in
contact with sulfide solution, this solution becomes dark in colour due to dissolution of mackinawite
in solution.
Figure 1: E-pH diagram E(NHE) Vs pH for S-H2O system as=0.2 gmole/lit
In the presence of oxidants e.g. oxygen or addition of sulfur, the nature of iron sulfides may change to
pyrrhotite and then to pyrite. Both of these sulfides have been observed to form under oxidizing conditions
and/or at high temperatures [13]. Also pyrrhotite and pyrite are protective type of sulfides [12, 13]. Perhaps
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that is the reason why on adding polysulfide (which form at oxidizing potentials in alkaline solutions) or
sulfur, corrosion rate of digester steel is found to decrease as it is protected by pyrrhotite and pyrite. Since at
potentials (~-0.5 VSHE), at which polysulfide are stable, γ- Fe2O3 is also stable (Figs. 1-4), it is also
expected to observe γ- Fe2O3 along with pyrrhotite and pyrite [9-11] in sulfide solutions up to ~
1200C.
c. The above aspects also forms the basis of protective digester by polarizing it anodically (anodic
protection) when steel gets passivated. While anodic protection systems for digesters have been
reported in literature [14-18] one report [4] that anodic protection for plant handling cooking liquor is
not feasible seems not correct.
It will be of interest to see nature of sulfides formed on steel (under anodic protection) and compare
them with their nature when formed on steel passivating by adding sulfur or polysulfide.
d. Once passivation is achieved, there is also a possibility of observing breakdown potential and hence
onset of localized corrosion. It should be tried to find upper limit of polysulfide in cooking liquor so
that steel does not experience localized corrosion.
e. Corrosion of increased sulfidity can be checked by comparing E-pH diagrams at 20% and 40%
sulfidity (Fig. 1 to 4). One finds an increased area of stability of sulfides at the cost of oxides. A higher
amount of sulfides in the corrosion [19-24] products is obviously going to retard the protective
characteristics of the rust layer provided that corrosion potential of steel does not shift much in anodic
direction.
Figure 2: E-pH diagram E(NHE) Vs pH for Fe- S-H2O system as=0.2 gmole/lit
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Figure 3: E-pH diagram E(NHE) Vs pH for S-H2O system as=0.7 gmole/lit
Figure 4: E-pH diagram E(NHE) Vs pH for Fe- S-H2O system as=0.7 gmole/lit
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4. Conclusion
The present study reports the effect of presence of sulfur species in cooking liquor on corrosion of digester and
other related machinery and that how the corrosivity of liquor is affected due to increased practice of pulping
wood chips at higher sulfidity. On the basis of E-pH diagrams drawn for S-H2O and Fe-S-H2O system
considering 20% and 40% sulfidity in the solutions, following conclusions can be drawn. In drawing
conclusions, help of E-pH diagrams for Fe-H2O [7] and Fe-S-H2O systems at 250C has also been taken
a. The alkalinity and sulfidity of cooking liquor and operating conditions in digester are such that pH
of liquor varies from ~ 14.05 to 11.69 as its temperature is raised from 250C to 1500C. Under these
conditions, sulfur containing species are HS- with small amount of S2- in reducing conditions and
polysulfide (Sn2-) and (S2O32-) under oxidizing conditions.
b. Presence of thionates and polysulfides increases corrosion with increased sulfidity and more
oxidizing conditions, the cooking liquor is expected to have more of polysulfide (higher values of n
in Sn2-) which increases corrosion further.
c. In cooking liquor, the corrosion products are expected products are expected to be mixture of
sulfides and oxide. With increased sulfidity, a larger parts of corrosion products are expected to be
consisting of sulfides. In reducing conditions, the sulfides could be mackinawite/troilite which are
non-protective type and hence may be responsible for increased corrosion attack on steel. While in
oxidizing conditions pyrrhotite and pyrite could be the iron sulfides which are protective type. As
such when polysulfide is increased in amount by more than 5 gpl, the corrosivity of cooking liquor
starts decreasing. However, beyond certain amount of polysulfide, the steel is likely to experience
localized corrosion.
d. The above aspects need to be verified by experiments which are expected to start soon.
e. The likelihood of formation of protective sulfides under oxidizing conditions gives a possibility of
protecting digester and related machinery of steel by polarizing it anodically (anodic protection).
Acknowledgements : The authors are thankful to Chairman, Prof. Kamal Ghansala and Dean Research,
Prof. Ankush Mittal, Graphic Era University for providing necessary facilities to complete this work.
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